KCM: Use the right memory context
    
    Inside the tevent request, we should use 'state' as the intermediate
    memory context and steal the result up to 'mem_ctx' on success.
    'mem_ctx' itself should only be used to create the tevent_req as the
    first thing during the request creation.
    
    However, this bug is not very severe as the mem_ctx was always the KCM
    operation memory context, so the memory was freed when the operation
    terminated.
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>